Advancements in Integrated Thermoelectric Power Generation and Water Desalination Technologies: A Comprehensive Review

Abstract

This paper reviews recent advancements in integrated thermoelectric power generation and water desalination technologies, driven by the increasing global demand for electricity and freshwater. The growing population and reliance on fossil fuels for electricity generation pose challenges related to environmental pollution and resource depletion, necessitating the exploration of alternative energy sources and desalination techniques. While thermoelectric generators are capable of converting low-temperature thermal energy into electricity and desalination processes that can utilize low-temperature thermal energy, their effective integration remains largely unexplored. Currently available hybrid power and water systems, such as those combining conventional heat engine cycles (e.g., the Rankine and Kalina cycles) with reverse osmosis, multi-effect distillation, and humidification–dehumidification, are limited in effectively utilizing low-grade thermal energy for simultaneous power generation and desalination, while solid-state heat-to-work conversion technology, such as thermoelectric generators, have low heat-to-work conversion efficiency. This paper identifies a key research gap in the limited effective integration of thermoelectric generators and desalination, despite their complementary characteristics. The study highlights the potential of hybrid systems, which leverage low-grade thermal energy for simultaneous power generation and desalination. The review also explores emerging material innovations in high figure of merit thermoelectric materials and advanced MD membranes, which could significantly enhance system performance. Furthermore, hybrid power–desalination systems incorporating thermoelectric generators with concentrated photovoltaic cells, solar thermal collectors, geothermal energy, and organic Rankine cycles (ORCs) are examined to highlight their potential for sustainable energy and water production. The findings underscore the importance of optimizing material properties, system configurations, and operating conditions to maximize efficiency and output while reducing economic and environmental costs.

1. Introduction

Electrical production and freshwater supply are critical issues for modern societies, driven by rapid population growth. According to the United Nations, the global population is projected to rise at an annual rate of approximately 1.2%, reaching an estimated 8.9 billion by 2050 [1]. This anticipated growth underscores the importance of addressing the challenges associated with meeting the escalating demand for drinking water and energy, while renewable energy accounted for only 17.7% of the global final energy consumption in 2019, highlighting the need for a strong push in renewable energy installation.

The majority of electrical power in Australia is generated from fossil fuels [2]. According to data from the U.S. Energy Information Administration (EIA), the U.S. electrical generation mix in 2022 comprised 39.8% natural gas, 21.5% renewable energy, 19.5% coal, 18.2% nuclear energy, and 0.9% petroleum [3]. However, reliance on fossil fuels contributes to atmospheric pollution and poses a risk of depletion due to their non-renewable nature. Consequently, there is growing interest among researchers and industries in advancing power generation technologies that utilize renewable energy sources.

Another significant global challenge today is the scarcity of freshwater, with only 3% of the Earth’s surface water being freshwater, while the remaining 97% is seawater [4]. Water deficits are expected to rise substantially over the next 20 years from the current situation, resulting in approximately 60–75% of the global population will experience some kind of water stress each year [5]. As a result, seawater desalination has emerged as a critical approach to augment freshwater supplies, given that seawater constitutes most of the world’s water resources [6]. The International Energy Agency (IEA) [7] Water–Energy Nexus report shows that energy consumption across water treatment processes varies significantly by technology and feedwater salinity. For instance, seawater reverse osmosis (SWRO) can use up to seven times more electricity than brackish water reverse osmosis (BWRO). However, advancements in membrane technology could potentially lower SWRO’s energy requirement to 1.8 kWh/m³ [8,9]. Desalination processes are broadly categorized into thermal desalination and membrane desalination. Multi-stage flash distillation is an example of thermal desalination, while reverse osmosis represents a method of membrane desalination.

Waste heat generation is a common byproduct of many thermal and mechanical processes, and it is often released into the environment [10,11]. Globally, it is estimated that around 245 PJ of waste heat is generated annually [12]. Another study found that the technical potential of industrial waste heat in the European Union is around 304.13 TWh per year, particularly at temperatures between 100 °C and 200 °C [13]. Similarly, a study by Bianchi et al. [14] found that 49% of the energy used in industry in the EU was lost as waste heat, with 29% dissipated through exhausts or effluents and significant amounts at lower temperatures. Despite the availability of low-grade waste heat, technological limitations, high costs, and efficiency challenges restrict its usage in water desalination and power generation.

Researchers have studied use of waste heat in desalination processes like membrane distillation (MD), multi-stage flash (MSF), multiple-effect distillation (MED), and humidification–dehumidification (HDH) systems [15,16,17,18]. Power generation using waste heat has also been studied, with efforts focused on conventional thermodynamic cycles (e.g., Rankine and organic Rankine cycles) and newer technologies like solid-state and membrane-based power generation [19,20,21,22].

Power generation systems based on traditional thermodynamics power cycle, such as the Rankine cycle, are often costly due to their scalability, complexity, and higher maintenance due to many moving parts and inefficiencies at lower temperatures. In contrast, thermoelectric generators (TEGs) are compact, scalable with no moving parts, and hence require reduced maintenance compared to Rankine cycle heat engines. TEGs can produce power under very low temperature differences, and are reasonably efficient at temperatures below 150 °C [23]. Their simplicity and ability to be modularized makes integration with desalination technology attractive. This encouraged us to conduct a systematic review of the research published on the integration of thermoelectric generators and water desalination. This review paper makes a significant contribution by addressing the critical gap in the literature on the integration of thermoelectric power generation and water desalination technologies. While separate advancements have been made in both fields, this paper is the first to comprehensively examine the potential of combining thermoelectric generators (TEGs) with desalination processes, focusing on leveraging low-grade waste heat. The paper highlights emerging innovations, such as high figure-of-merit thermoelectric materials and advanced membrane technologies, to enhance system performance. Additionally, it explores hybrid systems integrating TEGs with renewable energy sources, such as concentrated photovoltaic cells and geothermal energy, to optimize efficiency and reduce environmental costs.

Table 1. Summary of the queries/keyword used to conduct the literature review.

	Year Range 2013–2024
Queries/Keywords	Science Direct Total Articles (Review Articles)	Scopus Total Articles (Review Articles)	Google Scholar Total Articles (Review Articles)
Desalination	15,326 (1252)	33,861 (2552)	338,000 (37,800)
Thermoelectric generator	2935 (159)	9908 (418)	24,000 (8920)
Combined power generation and Desalination	278 (17)	445 (27)	30,200 (17,000)
Thermoelectric AND Desalination	160 (19)	366 (32)	15,800 (2690)
Thermoelectric generator AND Desalination	89 (5)	127 (8)	8320 (1450)
Combined thermoelectric power generation AND desalination	23 (1)	39 (3)	13,000 (2180)

presents a summary of the queries that were used to conduct the literature survey, along with the number of papers. The Google Scholar data appear to be unreliable, as they report significantly higher article counts compared to Scopus and ScienceDirect, often by several orders of magnitude. This discrepancy suggests potential issues, such as duplicate entries, the inclusion of non-peer-reviewed sources, and broader indexing criteria, which may lead to an overestimation of the relevant literature.

Considering ScienceDirect and Scopus data, the literature review showed that there are several review papers on individual technologies of thermoelectric generators or membrane desalination for waste heat recovery, we have not found any review paper that focuses on combined thermoelectric power generation and desalination.

To bridge the identified gap, a systematic approach was adopted for this literature review, concentrating on the examination of efforts made in the past decade to integrate thermoelectric power generation with desalination technologies. For this, the articles were sourced from ScienceDirect and Scopus using the queries listed in Table 1. Inclusion criteria included studies published in the last decade (2013–2024), peer-reviewed articles, and research focused on modeling, system optimization, and experimental validations. Papers were screened through title, abstract, and full-text analysis to ensure relevance.

2. What Are Thermoelectric Generators (TEGs)?

A thermoelectric generator is a solid-state device that converts thermal energy to electrical energy through a temperature gradient across its sides [24], using the phenomenon known as the Seebeck effect. A voltage is produced when the two sides have a temperature difference. Materials that generate a higher voltage exhibit a higher Seebeck coefficient for a given temperature difference.

A thermoelectric generator (TEG) comprises P-type and N-type semiconductors, along with two ceramic substrates, as illustrated in Figure 1. The semiconductors are connected electrically in series, and thermally in parallel with the ceramic substrates [25]. Additionally, the ceramic substrates provide electrical insulation for the semiconductors. The advantages of TEG technology include the absence of moving parts [24,26] and working fluids, resulting in noiseless operation and minimal maintenance costs. However, the primary drawback of TEGs is their relatively low efficiency.

To enhance the efficiency of TEGs, researchers and industry are concentrating on factors such as material properties and the figure of merit (ZT). The figure of merit, ZT, represents the relationship between the thermoelectric material’s performance and the temperature difference across the two sides of the TEG. The value of ZT is crucial, as it can predict the efficiency and maximum power output of a TEG. ZT and TEG efficiency can be expressed by Equations (1) and (2), respectively.

$$ZT=\left[{\displaystyle \frac{{\left({\alpha }_p-{\alpha }_n\right)}^2}{{\left({\left({\lambda }_p{\rho }_p\right)}^{1/2}+{\left({\lambda }_n{\rho }_n\right)}^{1/2}\right)}^2}}\right]\left[{\displaystyle \frac{T_h-T_c}{2}}\right]$$

(1)

where ${\alpha }_p$ and ${\alpha }_n$ are the Seeback coefficients of P-type and N-type semiconductors, respectively. ${\lambda }_p$ and ${\lambda }_n$ are the thermal conductivities of P-type and N-type semiconductors, respectively.

${\rho }_p$ and ${\rho }_n$ are the electrical resistivities of P-type and N-type semiconductors, respectively.

$T_h$ and $T_c$ are the temperatures of the hot and cold side, respectively.

$${\eta }_{TEmax}={\displaystyle \frac{W_{elec}}{Q_h}}={\displaystyle \frac{\left(T_h-T_c\right)}{T_h}}{\displaystyle \frac{\sqrt{1+ZT}-1}{\sqrt{1+ZT}+\frac{T_c}{T_h}}}$$

(2)

These equations highlight the interdependence between the material properties (e.g., Seebeck coefficient, thermal conductivity, and electrical resistivity) and the operational conditions (e.g., temperature difference) that influence the performance and efficiency of TEGs. By optimizing these factors, it is possible to enhance both the efficiency and the power output of thermoelectric devices.

Thermoelectric Materials

Previous studies have demonstrated that the figure of merit (ZT) significantly improves when materials are developed in nanostructured form. A solvothermal approach using Bi(NO₃)₃·5H₂O and TeO₂ has been proposed to synthesize Bi₂Te₃ single-crystal nanostructures, utilizing HNO₃ as a key reagent. This method involves controlled co-precipitation, magnetic stirring, and reduction with NaBH₄, followed by calcination and hydrogen treatment to produce oxide-free Bi₂Te₃. Mamur, H. et al. have suggested that further research should explore optimizing thermoelectric properties by enhancing the Seebeck coefficient while maintaining low thermal conductivity and high electrical conductivity [27].

Sharma et al. have found that lead telluride (PbTe) demonstrates exceptional thermoelectric performance, achieving a high figure of merit (ZT) of 2.5, the highest among thermoelectric materials [28]. This superior efficiency is attributed to its optimal carrier concentration and low lattice thermal conductivity. Notably, p-type PbTe alloys outperform their n-type counterparts, likely due to higher valley degeneracy and a large Grüneisen parameter. While band engineering has been less effective in n-type PbTe, strategies such as vacancy reduction and hierarchical structuring may enhance performance. These advancements in PbTe could provide valuable insights for optimizing other thermoelectric materials and improving energy conversion efficiency.

Recent studies recommend silicon–germanium (Si-Ge) alloys in high-temperature thermoelectric applications [29]. These materials exhibit a unique combination of thermal and electrical properties, including tunable energy band gaps, high solubility for dopants, and reliable performance in extreme conditions. Despite their success, the reduction in available thermal inventory in next-generation RTGs necessitates improvements in ZT for both n- and p-type compositions. Emerging materials such as skutterudites and Zintl phases offer potential alternatives, though extensive testing is required before replacing Si-Ge in critical missions with decades-long operational lifespans.

Thermoelectric materials exhibit unique advantages and limitations, with recent research focusing on optimizing their efficiency through structural modifications. Rogl et al. demonstrated that refining grain size in skutterudites significantly enhances thermoelectric performance by reducing lattice thermal conductivity, achieving record-high ZT values of 1.3 and 1.6 for p-type and n-type materials, respectively [30]. Zhao et al. highlighted the anisotropic thermoelectric behavior of SnSe, where ultralow lattice thermal conductivity along the b-axis resulted in an unprecedented ZT of 2.6 [31]. This underscores the importance of intrinsic material properties, such as anharmonicity, in achieving high efficiency without nanostructuring. Inoue et al. investigated SiC-nanoparticle/Mg₂Si composites, demonstrating improved fracture toughness while maintaining electrical conductivity. However, a trade-off was observed as excessive SiC additions led to reduced ZT values [32].

These studies emphasize that optimizing thermoelectric materials requires balancing structural modifications, thermal conductivity, and mechanical stability to maximize efficiency for practical applications. It is evident that thermoelectric generators (TEGs) utilize a variety of materials, each suited for different temperature ranges and applications. These materials, with their unique figures of merit and characteristics, are critical in determining the efficiency and suitability of TEGs for specific applications. The choice of material directly influences the efficiency of energy conversion, which is vital for harnessing waste heat and other energy sources in various industries. To provide a simpler visual understanding of the various materials employed in TEGs, their operating temperature ranges, applications, figure of merit, and key characteristics, a summary of different thermoelectric materials is presented in

Table 2. Summary of different thermoelectric materials.

Material	Operating Temperature Range (°C)	Figure of Merit (ZT)	Characteristics and Applications	References
Bismuth Telluride (Bi2Te3)	0–150	~1 at 300 K	High thermoelectric performance at room temperature, but limited thermal stability at higher temperatures. Effective for low-temperature applications. Cooling devices, low-temperature power generation.	[27]
Lead Telluride (PbTe)	300–600	~0.8–2.5 at 600 K	Suitable for mid-temperature applications with good thermoelectric performance. Environmental concerns are due to lead toxicity. Waste heat recovery, mid-temperature power generation.	[28]
Silicon–Germanium (SiGe)	500–1000	~0.5–0.7 at 900 K	Excellent thermal stability for high-temperature applications but lower efficiency compared to other materials. Cost remains a limiting factor. High-temperature power generation, space applications.	[29]
Skutterudites	300–700	~1.0–1.5 at 700 K	High electrical conductivity with complex synthesis. Promising for automotive waste heat recovery, but challenging scalability. Waste heat recovery, automotive applications.	[30]
Tin Selenide (SnSe)	300–800	0.8 to 2.6 at 923 K	Outstanding thermoelectric performance with high ZT value, but anisotropic properties and challenging large-scale production. Power generation, waste heat recovery.	[31,33]
Magnesium Silicide (Mg2Si)	300–600	~0.6–1.45 at 600 K	Abundant, cost-effective, and environmentally friendly. Efficiency improvements are required for competitive performance. Automotive waste heat recovery, power generation.	[32,34]

.

Applications of TEGs range from low-temperature energy harvesting to high-temperature waste heat recovery. For instance, in space missions, radioisotope thermoelectric generators (RTGs) have long been used to power spacecraft, such as in the Apollo and Voyager missions [26,35]. On Earth, industrial applications focus on utilizing waste heat from processes to improve overall system efficiency [36,37,38,39,40]. TEGs also hold promise for powering sensors in remote locations, reducing the reliance on traditional battery power sources [41,42,43,44]. In addition to their standalone applications, the efficiency of thermoelectric generators (TEGs) can be significantly enhanced when integrated with other power generation technologies. An example of such integration is demonstrated in the work of Ma et al. [45], where a combined system of a concentrated photovoltaic cell (CPV) and a two-stage thermoelectric generator (TTEG) was developed, as depicted in Figure 2. This combined system not only generates electricity from solar energy, but also recovers waste heat to boost overall energy conversion efficiency. In this configuration, a concentrator positioned above the PV module focuses sunlight onto the module, converting solar irradiance into electrical power while also producing waste heat. The waste heat is absorbed by the hot side of the TTEG, with the cold side being the surrounding environment. A mathematical model was employed to predict the power output and system efficiency under various operating conditions. The results indicate that both the maximum energy efficiency (MEE) and the maximum power density (MPD) of the combined system are superior to those of the CPV system alone [45].

Thermoelectric generators (TEGs) are produced using various materials and technologies, each suited for specific applications. The most commonly used thermoelectric devices are ceramics-based, and use bismuth telluride as the base semiconductor material for medium-temperature (below 250 °C) applications [46]. In recent years, flexible thermoelectric devices have gained attention. They are made from materials like polyimide, cellulose fibers, and fabric as the support structure, while the common semiconductor material is still bismuth telluride [47]. For applications with temperatures above 250 °C, lead telluride (500 °C to 800 °C) [48] is used as the material. To ensure high electrical performance, TEG materials must have high output voltage, high electrical power, and low internal electrical resistance.

3. Membrane Distillation (MD)

Membrane distillation (MD) is a separation process that selectively allows water vapor to pass through a membrane. Due to a pressure difference, water vapor is condensed and collected on the permeate side as the freshwater. The method of collection on the permeate side differentiates the four primary MD configurations: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD), and sweeping gas membrane distillation (SGMD) [49]. Beyond MD configurations, the membrane itself plays a crucial role in the process, with ongoing research addressing challenges such as fouling, membrane wetting, and scaling. This section will discuss the various MD configurations, membrane types, membrane development, and spacer development.

3.1. Membrane Distillation Configurations

3.1.1. Direct Contact Membrane Distillation (DCMD)

This method represents the simplest form of membrane distillation and requires minimal equipment [50]. The membrane separates two compartments: the feed side, containing hot liquid, and the permeate side, containing cold liquid. Water vapor passes through the membrane and subsequently condenses into the liquid on the permeate side, mixing with the cold liquid. Direct contact membrane distillation (DCMD) can achieve high flux [51,52] (flux means the rate of permeate production per unit area of the membrane); however, it exhibits lower thermal efficiency compared to other configurations, due to conductive heat loss [52].

3.1.2. Air Gap Membrane Distillation (AGMD)

In air gap membrane distillation (AGMD), water vapor passes through the membrane and condenses on the other side, collected in the air gap. The air gap, positioned between the membrane and the cooling plate, reduces conductive heat loss in this configuration [51]. An additional advantage of AGMD is its ability to eliminate volatile compounds from aqueous solutions [53]. However, this configuration typically results in lower permeate flux compared to other membrane distillation configurations, due to the mass transfer resistance introduced by the air gap [51].

3.1.3. Vacuum Membrane Distillation (VMD)

In vacuum membrane distillation (VMD), a vacuum pump maintains the driving force on the feed side, ensuring that the pressure remains below the liquid’s saturation pressure [51]. Subsequently, water vapor is condensed outside the membrane module. A key advantage of VMD is the negligible conductive heat loss [53]. However, the likelihood of membrane wetting increases under vacuum conditions [51].

3.1.4. Sweeping Gas Membrane Distillation (SGMD)

In this configuration, an inert gas is utilized to transport water vapor, which is then condensed outside the module. Due to the small volume of vapor mixed with a large volume of sweep gas, a sizable condenser is required outside the module, leading to increased costs [51]. However, using inert gas eliminates the risk of membrane wetting in the sweep gas membrane distillation (SWMD) process [51].

Figure 3 and

Table 3. Advantages and disadvantages of four membrane distillation configurations [54,55,56,57].

	DCMD Direct Contact Membrane Distillation	AGMD Air Gap Membrane Distillation	VMD Vacuum Membrane Distillation	SGMD Sweeping Gas Membrane Distillation
Unique feature	Feed and permeate in direct contact with the membrane	Air gap between membrane and condensation surface	Vacuum applied to the permeate side for vapor removal	Gas sweeps vapor from membrane surface
Advantage	Simplest configuration, high flux	Less conductive heat loss, volatile compounds elimination	Conductive heat loss is negligible	No membrane wetting
Disadvantage	Highest heat loss	Lower permeate flux	Membrane wetting	Cost of sweep gas
STEC	1500 to 4580 kWh/m3	~1110 kWh/m3	160 to 3550 kWh/m3	1090 to 1450 kWh/m3

present the various membrane distillation (MD) configurations, along with their respective advantages and disadvantages.

3.2. Heat and Mass Transfer in the MD Process

In the MD configuration, both mass and heat transfer processes occur simultaneously. The mass flow rate through the permeate side depends on the surface temperatures on both sides of the membrane. The bulk feed temperature (Tf) decreases to the bulk permeate temperature (Tp) due to the combined effects of boundary layer resistance, membrane resistance, and resistance within the membrane pores. The heat transfer, mass transfer, and temperature profile within this configuration, specifically for direct contact membrane distillation (DCMD), are illustrated in Figure 4.

Heat transfer through the feed side (${\dot{Q}}_f$) depends on bulk feed temperature ($T_f$) and membrane feed temperature ($T_{mf}$), which can be determined by Equation (3)

$${\dot{Q}}_f={Ah}_f\left(T_f-T_{mf}\right)$$

(3)

where $h_f$ is the convective heat transfer coefficient of the feed solution. A is the area of heat transfer.

Also, convective heat transfer through the permeate side (${\dot{Q}}_p$) depends on bulk permeate temperature ($T_p$) and membrane permeate temperature ($T_{mp}$), and can be determined using the following equation (Equation (4)).

$${\dot{Q}}_p={Ah}_p\left(T_{mp}-T_p\right)$$

(4)

where $h_p$ is the convective heat transfer coefficient of permeate solution.

The rate of heat transfer of membrane (${\dot{Q}}_m$) is based on conduction heat loss (${\dot{Q}}_{cm}$) and heat transfer of vapor through the membrane (${\dot{Q}}_v$). Membrane heat transfer can be calculated as follows (Equation (5)):

$${\dot{Q}}_m={\dot{Q}}_{cm}+{\dot{Q}}_v=A{k}_{mt}\left({\displaystyle \frac{T_{mf}-T_{mp}}{{\delta }_m}}\right)+AJ∆H_{v,w}$$

(5)

where $k_{mt}$ is the thermal conductivity of the membrane, ${\delta }_m$ is the thickness of the membrane. $J$ and $∆H_{v,w}$ are the mass flux and vapor enthalpy of water, which can be described by Equations (6) and (7).

$$∆H_{v,w}=1.7535T_{mf}+2024.3$$

(6)

$$J=C\left(P_{v,sw}-P_{v,w}\right)$$

(7)

where $C$ is the membrane distillation coefficient.

$P_{v,w}$ and $P_{v,sw}$ are vapor pressure of water and seawater, which can be estimated by Equation (8) and Equation (9) [58], respectively.

$$P_{v,w}=exp\left({\displaystyle \frac{a_1}{T_{mp}}}+a_2+a_3{T}_{mp}+a_4{T}_{mp}^2+a_5{T}_{mp}^3+a_6\times ln\left(T_{mp}\right)\right)$$

(8)

$$P_{v,sw}=P_{v,w}\times exp\left(-4.5818\times {10}^{-4}S-2.04431\times {10}^{-6}{S}^2\right)$$

(9)

where $S$ is salinity $\left(0–160\mathrm{g}/\mathrm{k}\mathrm{g}\right)$.

$$\begin{array}{c}{a}_1=-5800,a_2=1.3915,a_3=-4.8640\times {10}^{-2}\\ a_4=4.1765\times {10}^{-5},a_5=-1.4452\times {10}^{-8},a_6=6.5460\end{array}$$

The membrane distillation coefficient equation can be chosen depending on the membrane pore diameter and Knudsen number $\left(Kn\right)$, which is the ratio of the gas mean free path $\left(\lambda \right)$ to the membrane pore diameter $\left(d_p\right)$ [59].

If $Kn>1,or{d}_p<\lambda$, the predominant mass transfer mechanism is Knudsen diffusion, and the coefficient is $C_k$,

$$C_k={\displaystyle \frac{2{\epsilon }_{m}r}{3RT\tau {\delta }_m}}{\left({\displaystyle \frac{8RT}{\pi M}}\right)}^{0.5}$$

(10)

If $Kn<0.01,or{d}_p>100\lambda$, the predominant mass transfer mechanism is molecular diffusion, and the coefficient is $C_m$,

$$C_m={\displaystyle \frac{{\epsilon }_{m}PDM}{P_{a}RT\tau {\delta }_m}}$$

(11)

If $0.01<Kn<1,or{\lambda <d}_p<100\lambda$, the mass transfer mechanism is combination of Knudsen–molecular diffusion, and the coefficient $C_{km}$ is,

$$C_{km}={\displaystyle \frac{1}{RT{\delta }_m}}{\left({\displaystyle \frac{3\tau }{2{\epsilon }_{m}r}}{\left({\displaystyle \frac{\pi M}{8RT}}\right)}^{0.5}+{\displaystyle \frac{P_a\tau }{{\epsilon }_m{P}_t{D}_{w-a}}}\right)}^{-1}$$

(12)

where $R,{\delta }_m,M{,\tau ,P}_a,{\epsilon }_m$ are gas constant, membrane thickness, molecular weight of water, membrane tortuosity, entrapped air pressure, and membrane porosity, respectively. Also, $P_t{D}_{w-a}=\left(1.895\times {10}^{-5}\right)T^{2.072}$.

At a steady state, the amount of heat transferred to each region is equal [60,61].

$${\dot{Q}}_f={\dot{Q}}_m={\dot{Q}}_p$$

(13)

The total heat transfer coefficient can be determined by Equation (14).

$$H={\left({\displaystyle \frac{1}{h_f}}+{\displaystyle \frac{1}{\frac{k_m}{{\delta }_m}+\frac{J∆H_{v,w}}{\left(T_{mf}-T_{mp}\right)}}}+{\displaystyle \frac{1}{h_p}}\right)}^{-1}$$

(14)

The surface temperature of the feed and permeate sides cannot be measured experimentally [50,60]. Therefore, a mathematical model has been developed to predict these temperatures using Equations (15) and (16).

$$T_{mf}={\displaystyle \frac{h_m\left(T_p+\frac{h_f}{h_p}{T}_f\right)+h_f{T}_f-J∆H_{v,w}}{h_m+h_f\left(1+\frac{h_m}{h_p}\right)}}$$

(15)

$$T_{mp}={\displaystyle \frac{h_m\left(T_f+\frac{h_p}{h_f}{T}_p\right)+h_p{T}_p+J∆H_{v,w}}{h_m+h_p\left(1+\frac{h_m}{h_f}\right)}};h_m={\displaystyle \frac{k_m}{{\delta }_m}}$$

(16)

The temperature polarization coefficient (TPC) is a parameter used to model boundary layer resistance in membrane processes. It is defined as the ratio of the temperature difference across the membrane surface to the temperature difference in the bulk fluid. The TPC value ranges from 0 to 1. A TPC approaching 0 indicates a significant loss of driving force and a limitation in heat transfer across the membrane, suggesting that thermal polarization is dominant. Conversely, a TPC close to 1 suggests that the process is primarily limited by mass transfer, with minimal impact from temperature polarization effects.

$$TPC={\displaystyle \frac{T_{mf}-T_{mp}}{T_f-T_p}}$$

(17)

The gain output ratio (GOR) represents the system’s thermal efficiency [62]. High GOR values mean high system efficiency [50].

$$GOR={\displaystyle \frac{A\times J\times ∆H_{v,w}}{{\dot{Q}}_f}}$$

(18)

Specific energy consumption (SEC) is the energy required to produce 1 m³ of distilled water [50].

$$SEC={\displaystyle \frac{{\dot{Q}}_f}{A\times J}}$$

(19)

3.3. Membrane

The choice of membrane type and material plays a critical role in determining the performance and efficiency of MD processes. The importance of membrane material selection has been addressed by several studies exploring innovative membrane designs, fabrication techniques, and strategies to improve long-term performance in MD applications.

Li et al. [63] have studied the fabrication and performance of PVDF nanofibrous composite membranes for MD applications. They have not conducted a study on long-term membrane stability under harsh operational conditions, such as high salinity and temperature fluctuations. Future research should explore advanced surface modification techniques to enhance anti-fouling and anti-wetting properties.

Essalhi et al. [64] investigated the performance of electrospun PVDF nanofibrous membranes (ENMs) in DCMD for desalination. The authors observe that thicker membranes exhibit lower permeate flux, the nonlinear relationship between thickness and flux could benefit from further clarification, especially in terms of heat conduction dynamics. Additionally, the lack of significant changes in fiber diameter and void fraction across varying electrospinning times raises questions about the robustness of these parameters in influencing membrane performance. The comparative analysis against other PVDF membranes is helpful, but lacks a detailed exploration of potential trade-offs, such as energy efficiency versus flux.

Drioli et al. [65] have conducted and studied hallow fiber membrane properties with DCMD system performance, demonstrating improved flux in membranes with optimized morphology and additive combinations. On the other hand, the influence of long-lived operation on membrane fouling and stability remains not discovered. Future research should investigate the scalability of the production process and explore novel eco-friendly additives and further optimizing membrane structures can improve DCMD efficiency whilst minimizing the environmental impact.

Hou et al. [66] studied the role of hydrophobic nanoparticles in improving hallow fiber membrane properties for DCMD configuration. They have not studied the scalability of the fabrication process or address potential fouling issues under prolonged operational conditions. Future studies should focus on optimizing the nanoparticle dispersion technique to ensure uniform membrane characteristics and minimize thermal losses. Moreover, conducting comparative studies with different hydrophobic additives could reveal more efficient membrane configurations. Investigating the impact of various operational parameters, including salinity variations and feed flow rates, would further support the development of robust, high-efficiency membranes for industrial-scale desalination.

Duong et al. [67] have investigated an effective hybrid ultrafiltration (UF)/RO-AGMD approach for coal seam gas (CSG) brine treatment, achieving high water recovery with stable distillate production. However, the low permeate flux and potential long-term scaling risks require further investigation. Future studies should explore advanced anti-scaling strategies, real-time scaling monitoring, and novel membrane materials with higher permeability and fouling resistance. Additionally, the integration of renewable energy must be optimized to reduce specific energy consumption. Long-term pilot studies in varying field conditions will be essential to validate the system reliability and economic feasibility. Also, in 2016, Duong et al. [68] studied AGMD optimization for seawater desalination, particularly regarding energy efficiency and distillate production. Nevertheless, the research primarily focuses on single-pass operation, limiting an assessment of long-term membrane fouling and scaling effects. Furthermore, while the study acknowledges the influence of feed salinity, it does not explore membrane modifications to mitigate concentration polarization. Future analysis should investigate computational modeling combined with experimental validation, which could enhance predictive capabilities, optimizing the AGMD module for diverse operating conditions, including variable salinity and fluctuating thermal energy sources. Additionally, studies should be conducted on the feasibility of a large-scale AGMD system for industries.

Chen et al. [69] have studied the potential of tubular hydrophobic alumina membranes in VMD configuration; however, while the membrane exhibited high stability and performance, the study does not thoroughly address potential fouling effects on long-lasting operation, which could impact its long-term feasibility. In addition, the role of varying feedwater compositions on membrane performance remains unexplored. Future research should investigate membrane-fouling mechanisms and real-world applicability under different water conditions. Furthermore, comparative studies with alternative surface modifications and other hydrophobic coatings could enhance our understanding of performance trade-offs and long-term operational sustainability.

Pagliero et al. [70] have studied and demonstrated successful hydrophobization of tubular ceramic membranes and their stability at high temperatures, which broadens their application scope. However, the impact of long-term operation on membrane performance, fouling resistance, and mechanical integrity remains unaddressed. Moreover, the study lacks a direct comparison with commercial polymeric membranes under identical operating conditions. Future research should explore the scalability of these modified membranes and their economic feasibility. Investigating alternative functionalization techniques that enhance hydrophobic stability while maintaining membrane permeability could further improve efficiency and widen industrial adoption.

The literature review has shown many similarities and contradictions amongst researchers. The studies collectively explore membrane distillation (MD) performance enhancements but reveal critical research gaps. Li et al. [63] and Essalhi et al. [64] investigate PVDF nanofibrous membranes, but overlook long-term stability. Similarly, Drioli et al. [65] and Hou et al. [66] investigate hollow fiber membranes, yet both neglect operational longevity and scalability concerns. Duong et al. [67,68] contribute to hybrid UF/RO-AGMD and AGMD optimization, but this requires further exploration into fouling and scalability. Meanwhile, Chen et al. and Pagliero et al. focus on ceramic membranes, demonstrating thermal resilience, but lacking long-term fouling analysis and economic feasibility assessments. Collectively, future studies should emphasize long-term stability, scalability, and antifouling strategies to enhance membrane durability and efficiency across diverse desalination systems.

As summarized in

Table 4. Characteristics and performance of different membrane modules/materials.

Membrane Modules	Material	Membrane Physical Properties	Operating Conditions and Performance	Reference(s)
Flat Sheet	PVDF	Pore size: 0.2–0.45 µm Contact angle: 100–162° Thickness: 35–330 µm	DCMD, feed: 1–2 wt% NaCl, Tf: 60 °C, Tp: 21.1 °C, Qf: 4 cm3/s; flux: 7.5–32.8 L/m2h; rejection: 99.5–100%; LEP: 150–280 kPa	[63,64,71,72]
Hollow Fiber	PP/PVDF	Pore size: 0.3–0.35 µm Contact angle: 90–100° Thickness: 130–150 µm	DCMD, feed: 3.5 wt% NaCl, Tf: 80 °C, vf: 0.5 m/s; flux: 35–40 L/m2h; rejection: 99.9–99.99%; LEP: 300–320 kPa	[65,66,73,74,75,76]
Spiral Wound	PTFE/PP/PE	Pore size: 0.1–0.2 µm Contact angle: 105–115° Thickness: 120–130 µm	DCMD/AGMD, feed: city water, Tf: 70 °C, A: 14 m2; flux: 2.0–2.5 L/m2h; rejection: 99–99.9%; LEP: 150–250 kPa	[67,68,77,78]
Tubular	Alumina	Pore size: 0.2–0.8 µm Contact angle: 140–150° Thickness: 1500–2000	VMD, feed: 3 wt% NaCl, Tf: 70 °C, P: 5 kPa; flux: 28–60 L/m2h; rejection: 99.8–99.9%; LEP: 550–570 kPa	[69,70,79,80,81]

, membrane properties such as material, pore size, contact angle, and thickness significantly influence the performance of MD systems. For instance, flat sheet membranes made of PVDF demonstrate high flux rates and salt rejection, but operate at moderate pore sizes and LEP. In contrast, hollow fiber and tubular membranes offer higher flux rates under specific operating conditions, making them suitable for applications requiring higher salinity tolerance. Liquid entry pressure (LEP), along with flux and rejection rates, varies significantly depending on membrane material and configuration. For example, tubular membranes made of alumina exhibit high LEP values (550–570 kPa) and superior flux rates in VMD systems, making them ideal for high-salinity and low-pressure applications. Similarly, flat sheet membranes demonstrate versatility across multiple configurations like DCMD, balancing moderate LEP values with high rejection rates and scalability. The selection of the appropriate membrane type depends on the desired configuration (e.g., DCMD, AGMD, VMD) and specific application requirements.

4. Combined Power Generation and Water Desalination Technology

The integration of thermoelectric power generation with water desalination technologies has gained significant attention as a sustainable solution to address the dual challenges of energy and freshwater scarcity. Combined systems offer the potential to take advantage of low temperature renewable energy sources and waste heat to simultaneously produce electricity and freshwater in an environmentally friendly manner. This section reviews key advancements in combined power generation and desalination (CPD) technologies, focusing on experimental studies, mathematical models, and system optimizations reported in the literature. Each study is summarized to highlight its objectives, system design, performance outcomes, and identified limitations, providing insights into the current state and future directions of CPD research.

4.1. Experimental Studies and System Innovations

In 2023, Shoeibi and team [82] worked on a novel solar desalination system that integrated TEGs, solar reflectors, and thermal storage for combined power generation and water desalination. Researchers found that compared to the regular solar still, the proposed design increased productivity by 24.4% and was able to produce 0.796 L/m² of freshwater and 2.5 watts of electricity, which is 15.2% more electricity than a TEG-only system. They found that the system is also cost-effective, with a cost per liter of just USD 0.071, and has a potential of reducing CO₂ emissions by about 9.3 tons over its lifetime. They have recommended using materials with better heat conductivity, and also using machine learning to optimize the system. They also suggest studying other metals, like aluminum or copper, for better heat storage.

Moreover, Date et al. [83] have developed an integrated system for generating both electrical energy and freshwater utilizing low-temperature heat, as depicted in Figure 5. In this system, heat is transferred to a thermoelectric generator, and subsequently through a heat pipe into one side of a vessel to boil the seawater. On the opposite side of the vessel, heat pipes are employed to condense the water vapor, with the pipes immersed in a cooling water bath outside the vessel. Experimental investigations were conducted to validate the mathematical model. The results indicate that heat flux increases when the vessel operates under sub-atmospheric pressure. Additionally, reducing the pressure within the vessel was found to enhance the heat flux, which could otherwise be limited by the failure temperature of the thermoelectric generator.

Demir et al. [84] have proposed a cogeneration system that operates using solar radiation during the daytime and natural gas after sunset to generate electricity. As depicted in Figure 6, the system employs solar-driven volumetric pressurized air receivers (Brayton cycle) during the day and a combustion chamber during the night. The exhaust gases produced during electricity generation are directed to heat exchanger 1 (Rankine cycle) and a thermoelectric generator (TEG) to produce additional electricity, with seawater being circulated through the TEG by a pump. After passing through the steam turbine of the Rankine cycle, the wet steam is used to preheat seawater before it enters a flash distillation unit at low pressure. The pump is initially activated to reduce and maintain the pressure at 4.5 kPa. This study analyses and predicts energy efficiency, exergy efficiency, exergy destruction, distilled water production, power generation, and the conversion efficiency of the TEG using a computational model. With a 50% solar contribution, the system achieves overall exergy and energy efficiencies of 54.9% and 44.5%, respectively. Additionally, the TEG generates more than 32 kW of power with a heat-to-electricity conversion efficiency of 2.02%. Freshwater is produced at a rate of 3.36 kg/s. Furthermore, to enhance the freshwater production rate, the temperature of the wet steam should be increased.

Alzahmi et al. [85] have introduced a shell-and-tube heat exchanger (STHX) system, as depicted in Figure 7, which integrates a thermoelectric generator (TEG) with a reverse osmosis desalination process. In this system, seawater is initially heated to 25 °C as it passes through the STHX before entering the reverse osmosis unit, thereby enhancing the desalination efficiency. The system utilizes waste heat from thermal power plants operating at 300 °C. The study employs ANSYS FLUENT using the realizable k-ε turbulence model for simulations and compares the results with the well-established Bell–Delaware method, observing maximum deviations of 10.7% for the shell side and 2.6% for the tube side.

The computational fluid dynamics (CFD) results indicate that increasing the water flow rate leads to enhanced heat transfer and overall efficiency. Additionally, the study finds that increasing the salinity of the saline water at a constant flow rate results in higher outlet water temperatures, increased heat transfer rates, improved efficiency, and a greater number of transfer units (NTU). Conversely, higher salinity reduces the air outlet temperature and the rate of change while also decreasing the required pumping power.

Furthermore, the simulation results demonstrate that an increase in the temperature difference across the TEG enhances electrical power generation, although the temperature difference is limited by the material properties of the TEG. A notable drawback highlighted is the low efficiency of the TEG, with the simulation indicating an efficiency of only 5.5% at a temperature difference of 375 K.

The integrated system illustrated in Figure 8, which combines passive water desalination and a thermoelectric generator (TEG), was proposed by Myneni et al. [86]. The desalination process generates freshwater by reducing the saturation temperature through the creation of a vacuum. A negative head of 5 m creates a vacuum, which lowers the boiling temperature of the water. The thermoelectric generator unit (TEGU) is installed at the bottom of the evaporation chamber, where a heat source, such as waste heat or solar energy, is applied to the hot side of the TEG. This heat is transferred to the cold side, facilitating the boiling of seawater in the evaporation chamber. To maintain a low temperature on the cold side of the TEG, the temperature of the saline water is reduced. The study’s calculations indicate that a vacuum of 50 kPa can lower the saturation temperature by approximately 80 degrees Celsius. The temperature difference across the TEG generates voltage via the Seebeck effect. The experimental results presented in the study show that with heat inputs of 75 W, 100 W, and 150 W, the electrical outputs were 0.56 W, 0.73 W, and 0.94 W, respectively. These results indicate that an increase in temperature difference leads to an increase in power output.

The research conducted by Khanmohammadi et al. [87] presents an integrated system combining a thermoelectric generator (TEG) and reverse osmosis (RO) with a gas turbine–helium Brayton system, referred to as the GT-HBS/TEG-RO, as depicted in Figure 9 The core of this system is the gas turbine–helium Brayton cycle (GT-HBS). The integrated setup includes a solar tower unit, a helium Brayton cycle with atmospheric air, two organic Rankine cycles (ORCs) using R-123 as the working fluid, a thermoelectric generator (TEG), and reverse osmosis (RO) for desalination. The Engineering Equation Solver (EES) software was employed to develop a simulation code to analyze the influence of various parameters on both the thermodynamic behavior and efficiency of the system. The simulation results indicate an increase in power output by 941 W. The proposed system exhibits higher energy and exergy efficiencies compared to the standalone GT-HBS, with improvements of 3.7% and 5.2%, respectively. Additionally, an increase in the inlet temperature of the gas turbine leads to growth in the volumetric flow rate of freshwater, power output, energy efficiency, and exergy efficiency. Moreover, an increase in direct normal irradiation contributes to enhanced freshwater flow rate, energy efficiency, and exergy efficiency. However, a rise in the compressor pressure ratio initially increases the net output, energy efficiency, and exergy efficiency, but these eventually decline. The thermoelectric generator recovers a significant amount of rejected heat, which replaces the conventional condenser unit, positively impacting both energy and exergy efficiencies.

The research conducted by various teams in the field of combined power generation and desalination systems presents a range of innovative approaches. Shoeibi et al. [82] introduced a solar desalination system with TEGs, solar reflectors, and thermal storage, demonstrating a 24.4% increase in productivity, producing 0.796 L/m² of freshwater and 2.5 W of electricity. Assareh et al. [88] integrated solar and geothermal energy with TEGs, achieving 1158 kW power output and improved system efficiency. Similarly, Date et al. [83] optimized a low-temperature heat-based system, while Demir et al. [84] used solar and natural gas to power a cogeneration system with impressive freshwater production. Although these systems show potential for improving water and energy production, challenges such as low TEG efficiency and seasonal variations in energy sources remain, necessitating further research into material improvements and system optimization.

4.2. Multi-Generation Systems

A notable example of system integration is provided by Hashemian and Noorpoor [89], who developed a multi-generation plant powered by a solar/biomass hybrid system integrated with a thermoelectric generator unit. This system is designed to produce hydrogen, potable water, cooling loads, and electrical power, as illustrated in Figure 10. The integrated setup comprises parabolic trough collectors (PTC), a steam Rankine cycle (SRC), a combined ejector–absorption chiller (EAC) unit, a thermoelectric generator (TEG), a proton exchange membrane electrolyzer (PEME), and a multi-effect desalination (MED) unit. The study employs energetic, exergetic, exergoenvironmental, and exergoeconomic analyses, along with sustainability index estimation. Additionally, a genetic algorithm with a Pareto front approach is used for optimization, focusing on maximizing exergy efficiency and minimizing the total product cost rate. The optimization considers four design variables: the figure of merit for the TEG, turbine inlet pressure, PTC inlet pressure, and PTC length. The results indicate an energy efficiency of 76.12%, exergy efficiency of 14.24%, net power output of 33.03 W, potable water production rate of 16.09 m³/h, hydrogen production rate of 90.87 kg/h, and cooling load of 149.86 MW. Furthermore, the optimization results show an exergy efficiency of 15.59% and a total product cost rate of 1.42 USD/s.

The final example of an integrated system combined with TEG was conducted by Cao et al. [90]. This study investigated the effects of different working fluids on the organic flash cycle based on the Rankine cycle for power generation, freshwater production, and hydrogen generation. This study also explored four different refrigerants: R123 and R600 in organic flash, and R1234yf and R1234ze in an organic Rankine cycle (ORC). It was found that R600 improved the net power by around 6%. The mixed stream from the compressor and ORC turbine is supplied to the thermoelectric generators for power generation. The proposed system utilizes geothermal energy. The proposed system has many subsystems, and the overall energy conversion efficiency is not stated.

Assareh et al. [88] have investigated a combined solar and geothermal system designed for multipurpose applications, including cooling, electricity generation, hot water production, and desalination, as shown in Figure 11. The proposed system integrates a geothermal well, solar parabolic trough collectors (PTCs), a steam Rankine cycle (SRC), a reverse osmosis (RO) desalination unit, and an absorption chiller. To enhance electricity generation, a thermoelectric generator is employed in the SRC in place of a traditional condenser. The results demonstrate that the inclusion of the thermoelectric generator improves system efficiency and reduces the cost rate. Furthermore, the study examined the impact of factors such as solar collector area, solar radiation, and turbine inlet pressure on system efficiency. The system’s performance was also analyzed across all four seasons—fall, winter, summer, and spring—revealing that the highest electricity generation occurred during the summer. Specifically, on 15 July, the system produced 9146.7 kW of electricity and 240.226 m³ of freshwater.

To optimize the system, a genetic algorithm with a Pareto frontier was employed, focusing on seven design variables: collector outlet temperature, geothermal fluid temperature, total solar aperture area, evaporator pinch point temperature difference, turbine outlet pressure, turbine inlet pressure, and figure of merit. The objective functions were the exergy efficiency of the proposed system and the cost rate. The TOPSIS decision-making criterion was then used to select the optimal solution. The results indicated an exergy efficiency of 20.52% and a cost rate of USD 10.41 per GJ for the system.

Other example of a combined system which utilizes various system to produce drinkable water and electricity such as the Rankine cycle system, the organic Rankine cycle system, solar chimney for power generation, and multi-effect desalination (MED) and reverse osmosis for water desalination. For example, Bouzayani et al. [91] designed three cases of a power recovery unit with reverse osmosis (RO) for steam power plant (using a superheated Rankine cycle) and drinkable water production. The first two system cases, combine RO with a hydraulic turbine. The third case is RO combining with a pressure exchange unit (PES). The power plant only supplies power to pumps in the RO subsystem in the first case (case A), which is mechanical, while the second (case B) and third cases (case C) are mechanical and thermal, where heat rejection from a condenser of a power plant transfers to seawater in the RO subsystem. A parametric study is conducted to understand the impact of operation, including feed flow rate, salinity, operating pressure on energy efficiency, exergy efficiency, permeate quality, and permeate quantity, as well as the energy recovery system (hydraulic turbine or pressure exchanger), which is also investigated. The steady state model has been created by using thermodynamics laws for a superheated Rankine cycle of steam power plant and semi-empirical model for analyzing RO subsystem performance. The finding of the system presents that power plant mass flow rate must be higher than mass flow rate of seawater approximately 0.4% in all cases. In addition, mass flow rate of power plant rise the excess power, which is generated from vapor turbine; hence, the power can be converted into electrical power. Three systems can also generate approximately 855 KJ of excess power per unit mass flow rate of saltwater when two mass flow rates are identical. In terms of the RO subsystem, cases A-C can produce freshwater at 33.1%, 38.9% and 38.9% of the seawater mass flow rate, respectively, and the salt concentrations are 45, 1000, and 830 ppm, respectively. Additionally, as a result of energy and exergy flux of 3%, the steam power plant to seawater ratio shows that case B and C can produce more drinkable water than case A, approximately 18%, whilst the highest amount of power is generated from case C, and case B is lowest. Finally, the exergy destruction of the power plant is higher than RO subsystem by approximately ten times.

The comparison highlights variations in energy sources, system configurations, and optimization strategies, but all emphasize improving system efficiency and cost reduction through advanced modeling and optimization techniques.

4.3. Biomass and Waste Heat Integration

Chitgar and Moghimi [92] have proposed a fuel cell-based power, fresh water, and hydrogen generation system, as shown in Figure 12. They have used a parametric study approach to identify the boundary conditions that have the most influence on the system performance and conducted thermo-economic analysis. They have optimized the proposed system, and at optimum point they achieved an exergetic efficiency of 54.2%, and a total unit cost of products of USD 34.5/GJ. Further, they have achieved optimum fresh water production of 90.1 m³/h. When they considered the hydrogen production rate, the optimum value was 11.6 kg/h. This study used a genetic algorithm modeling approach for the optimization of the proposed system.

Safari and Dincer [93] have proposed an integrated system for power, fresh water, and hydrogen generation using biomass (sewage sludge) from a wastewater treatment plant, as depicted in Figure 13. The anaerobic digestion produces biogas, which is used to power an open-air Brayton cycle heat engine, and the waste heat from the Brayton engine is used to run an ORC heat engine and multi-effect desalination (MED) system. For the ORC, the working fluid used is R245fa. The desalinated water is used as the input to the proton exchange membrane electrolyzer for hydrogen generation. The waste heat from the ORC is also used to produce hot water for different applications. An analytical model of the proposed integrated system predicts that the system can generate 1102 kW of power and around 1 kg/s of freshwater, with an overall energy conversion efficiency of 63% and an exergetic efficiency of 40%.

Another example combining ORC with MED was proposed by Mohammed et al. [94] which recovered waste heat for cooling, heating power, electricity and freshwater production by utilizing an organic Rankine cycle (ORC)-based poly-generation system, as depicted in Figure 14. This research utilizes exergoeconomic and multi-objective optimization to analyze the possibility of the novel system. Additionally, the optimization is a method to compromise between the maximum thermal efficiency and minimum economic cost of the novel system via parametric adjustment. The study outcome indicates that optimized case improves 16% of electricity, 306.6% of cooling power and 50% of energy utilization factor (EUF), while it reduces 16% of the total product unit cost (cp,tot) and 9.5% of the electricity cost (Celec) compared to the base case. This means, in terms of electrical power and freshwater production, the system can generate 10 MW of power at an electricity cost of 0.8087 US cents/kWh whilst producing 38.51 m³/h at a total water cost of 0.4225 USD/m³.

Demir and Çıtakoğlu [95] designed a multipurpose combined system including electrical production, hydrogen production, water desalination, and cooling, as well as space heating. The system is designed to utilize waste heat from a marine diesel engine to serve an organic Rankine cycle (ORC) and absorption refrigeration cycle, as shown in Figure 15. The heat from condenser of ORC is used in the single-stage flash distillation unit (FDU) process to convert seawater into freshwater. Then, freshwater is supplied to the polymer electrolyte membrane (PEM) electrolyzer array to produce hydrogen production. Also, freshwater is stored in ballast tanks after ultraviolet (UV) treatment to decrease ballast water treatment unit costs. For this research, the thermodynamic and electrochemical models of the system have been conducted using Engineering Equation Solver (EES), and the parametric study has also been considered to understand the effect of parameters on the system operation. The outcome of this research presents the system performance of 25% energy efficiency and 13% exergy efficiency. The system can generate 659 kW net power and 0.535 kg/s freshwater. Additionally, hydrogen production can be produced at 306.8 kg/day using the PEM stack.

Hamrang et al. [96] have studied a simultaneous power and freshwater system based on the biomass gasification cycle. An open-loop Brayton cycle heat engine and a closed-loop steam-powered Rankine cycle heat engine has been used for power generation, while the waste heat from the Rankine cycle condenser is used to operate a multi-effect distillation system for freshwater production. The system performance has been investigated through an analysis of energy, exergy, and economics. The results show that for the base case, the proposed system could produce around 11.7 kg/s of freshwater and around 8.4 MW of power with exergetic efficiency of around 46%. Despite the varied approaches, challenges remain in optimizing energy efficiency and minimizing costs across these multi-generation systems.

4.4. Solar and Wind Energy Combined Systems

Zhu et al. [97] have proposed a solar energy-based cascaded system that can produce power, a cooling effect, and freshwater at the same time. The parabolic trough solar thermal collectors are used to supply thermal energy to organic Rankine cycle heat engine with an ejector-based refrigeration cycle and steam power cycle coupled with a vapor compression multi-effect desalination system. They show that the baseline performance of the system would have an exergetic efficiency of around 14.22% and a payback period of 5.32 years.

Al-Nimr et al. [98] designed an integrated system, illustrated in Figure 16, comprising a solar dish Stirling engine unit, a thermoelectric cooler (TEC), and a vacuum chamber for saline water evaporation and freshwater condensation. In this system, saline water from a storage tank absorbs heat from both the solar dish Stirling engine and the hot side of the TEC. The heated saline water is then directed to the evaporator chamber, where it evaporates and subsequently condenses in the condenser chamber connected to the cold side of the TEC. The remaining saline water from the evaporator chamber is returned to the storage tank, while the condensed freshwater is collected and stored in a freshwater tank. A mathematical model was developed and validated against published data. The results indicate that system performance improves with lower wind speed, larger solar dish diameter, higher solar dish reflectivity, lower mass flow rate of saline cooling water, and higher solar radiation intensity. Although hydrogen provides the highest power output as a working fluid in the Stirling engine, helium is considered safer. Additionally, an increase in solar radiation intensity negatively impacts the TEC, but positively affects the vacuum evaporator. The study also found that a combination of preheating and the TEC module yields better performance in terms of freshwater production and efficiency compared to using either component alone. The system achieves a net power output of 502.8 W, with the solar dish Stirling engine generating 1275 W, while the TEC and vacuum pump consume 506 W and 266.2 W, respectively.

Solar power is widely used for power generation, as will be seen in the following example of an integrated system. Yari et al. [99]. have conducted cogeneration which consists of a solar still, semitransparent, photovoltaic, and evacuated tube collector in a natural mode, as depicted in Figure 17. This research studied various aspects, including the type of photovoltaic module, number of evacuated tubes, different basin depths, thermal efficiency, as well as distilled water and electrical production. The research outcome illustrates that type of PV modules do not have any effects on distillate yield. An increased number of tubes have a positive impact on distillate yield while having a negative impact on energy and exergy efficiencies as thermal losses increase. Also, solar radiation, the PV module, and the temperature of the PV module have an impact on the electrical power production. Additionally, maximum water production is 4.77 kg/m² a day, with 0.07 m basin depth and 30 tubes, while maximum instantaneous and daily electricity are 70.48 W/m² and 483.2 Wh/m², respectively, with a HIT photovoltaic module type. The maximum daily energy and exergy efficiencies are 6.86% and 16.65%, respectively, with 10 tubes and 0.07 m basin depth.

Zheng et al. [100] also took advantage of solar power to generate electricity, freshwater, and hydrogen production. Zheng et al. investigated a hybrid energy, water, and hydrogen system that integrates a solar-powered Brayton cycle with desalination and hydrogen production. They propose to use reverse osmosis for water desalination and PEM electrolyzer for hydrogen production. The low-temperature thermal energy is used to operate an organic Rankine cycle heat engine. They conducted techno-economic analysis of this hybrid system and established its feasibility. Their modeling results show that the major exergy destruction happens in the solar tower and receiver. They conducted a case study for San Francisco, with peak power generation of around 242.15 MWh in summer and around 143.11 MWh in winter.

Demir and Dincer [101] have proposed a novel integrated solar energy system with thermal storage and multi-stage flash distillation (MFD) for distilled water and electrical power production. The system begins with solar tower with a volumetric solar receiver that heats the molten salt and the molten salt flow through a heat exchanger to generate steam for the Rankine cycle in order to generate electricity. A proportion of the molten salt flows into hot storage tank to supply energy when direct normal irradiance (DNI) level is low. MFD subsystem uses seawater, which is heated by a saturated steam–water mixture coming from the steam turbine to produce drinkable water. The study analyses the system performance via the Engineering Equation Solver (EES). The outcome shows that it obtains 13.5 MW of electrical power and 3958 tons per day of freshwater production. In addition, the overall energy and exergy efficiencies of the system are 19.9% and 46.5%, respectively, as well as 73% of the overall exergy destruction being represented by a solar receiver and a steam generator. In 2021, Demir and Dincer [102] also proposed a similar system, as shown in Figure 18, to generate and produce electricity, freshwater, and ammonia by solar thermal energy. The system begins with a solar thermal subsystem which collects solar thermal energy, which is used to charge molten salt at a thermal energy storage. Then, molten salt flows across a heat exchanger and generates steam that supplies turbine to produce electricity. The heat from the exhaust steam of the turbine is the thermal energy source for a 20-stage multistage flash distillation (MFD) unit where seawater is feedwater. A proportion of the distilled freshwater flows through the PEM electrolyzer array to generate hydrogen as a primary substrate to produce ammonia via the Haber–Bosch process. This research analyses the thermodynamic behavior of the system via the Engineering Equation Solver (EES) and Aspen Plus software packages. The findings illustrate that the overall energy and exergy efficiency of integrated system are 70.3% and 12.1%, respectively. Also, electrical, freshwater, and ammonia production are 17.6 MW, 143.97 kg/h and 0.85 kg/s, respectively. Another study on this subject indicates that the attached air separation unit has less than a 0.3% effect on the energy and exergy efficiencies of the integrated system compared to the system without it.

Azad et al. [103] have proposed optimization of solar chimney integrated with water desalination using neural networks, as shown in Figure 19. The proposed system utilizes a solar chimney effect to create a natural draught that picks up moisture from the pool of saline water, and the rising air turns a turbine to generate power. The moisture from the air is captured in the condenser as freshwater. A genetic algorithm is used for optimization, and the optimized system is estimated to generate around 719 kW and 14.28 kg/s of fresh water for a solar collector diameter of around 1384 m and chimney height of around 167 m.

Zuo et al. [104] have also designed an integrated system that combines a wind supercharged solar chimney power plant and seawater desalination (WSSCPPCSD). The system consists of a wind pressure ventilator and a solar chimney power generation system. The numerical simulation is utilized to investigate and study the effects of the wind pressure ventilator with unsteady state condition and steady state condition on the solar chimney power generation system. This study also compares the results with a solar chimney power plant (SCPP) and a solar chimney power plant combined with seawater desalination (SCPPCSD). For comparison, it was found that 45% of shaft power of WSSCPPCSD is higher than SCPPCSD, which causes increased power generation and freshwater production. Also, the result shows that the wind pressure ventilator has a positive effect on power generation and freshwater production as it supplies a negative pressure of 64.5 Pa at the chimney outlet. In addition, the system can produce additional 14.7 kW, and 30 g/h every hour at 800 W/m². In 2020, Zuo et al. [105] developed two proposed systems for power generation and water desalination. They are a wind supercharging solar chimney power plant combined with seawater desalination and waste heat (WSCPPDW), as shown in Figure 20, and a solar chimney power plant combined with seawater desalination and waste heat (SCPPDW). A spiral exhaust gas heating channel (SGC) is equipped with two designed systems, whilst the main difference in the systems is the presence or absence of a wind surcharge device. The mathematical model and experiment were conducted to assess the outcomes under different parametric settings. The result illustrates that an increasing chimney height and temperature of flue gas both improve the overall system performance and decrease the seawater thickness, also enhancing the overall output behavior under sufficient irradiance conditions. A mass flow rate of flue gas rise under solar irradiance causes an increase in electrical power generation; however, comprehensive efficiency and distilled water production have a negative impact. By comparison, WSCPPDW performance was better than SCPPDW performance. As with high-altitude wind, the comprehensive energy utilization efficiency of WSCPPDW rose by 15.4%. In addition, electrical power generation, yield of freshwater production and comprehensive efficiency of WSCPPDW were 193.7 kW, 17.2 ton/h, and 13.5%, respectively.

Similarly, Chitsaz et al. [106] have investigated the use of CO₂ in a Brayton cycle powered by biogas generated using the biomass gasification process. They have also proposed combining the waste heat from the Brayton cycle heat engine to preheat the saline water that is fed to the humidification–dehumidification desalination. This trigeneration system has been modeled and optimized using multi-objective surface response method. The results show that the freshwater production improved as the biomass feed rate and steam to biomass ratio increased, while the freshwater production decreased when the pressure ratio was increased. The optimized design of the proposed system was predicted to produce around 172 kW of power and around 778 kg/h of freshwater when the biomass feed rate was 1.2 mol/s and the biomass to steam ratio was 1.5.

Final example of combined system, Li et al. [107] have conducted a spontaneous sustainable multifunctional transpiration generator (SSMTG), as depicted in Figure 21, for electrical power, salt, and freshwater production. This generator is designed to improve the performance of power generation driven by evaporation, as well as the continuity of power generation. Controlling the illumination area of sunlight can maintain a dry/wet interface on a carbon black-coated cotton cloth, which helps to continuously generate electrical power. Also, water is spontaneously moved by capillary action via a cotton rope. The outcome of the single SSMTG indicates that the configuration operated for more than 24 h and produces 0.825 g and 29 mL salt and freshwater, respectively. In terms of the electrical result, the configuration can generate approximately 0.4 V of open-circuit voltage and 23.6 µA of short-circuit current, with SSMTG size 9 cm × 3 cm × 0.01 cm.

Table 5. Characteristics and performance of hybrid renewable energy technologies integrating power generation and desalination systems.

Technology (Study Type)	Energy Source/Temperature	Power Generation Capacity	Feed Source/Salinity	Desalination System Performance	Freshwater Generation Capacity	Reference
Kalina cycle + HDH-D + TEG	Solar pond 25–96 °C	8.75 kW	35,000 ppm	GOR 0.5 to 1.8	0.152 to 0.586 m3/h	[108]
HDH-D + TEG	Solar pond 25–96 °C	3.5 kW	Seawater 35,000 ppm	GOR 1.56	4.5 m3/day	[109]
HDH-D + TEG	Solar parabolic collectors (nanofluid) 120–140 °C	1.785 kW	35,000 ppm	Combined GOR 1.2 to 1.6	0.36 m3/h	[110]
Brayton cycle + Rankine cycle + ORC + RO + TEG	Heliostat solar collector 80–800 °C	3.59 to 9.48 MW	35,000 ppm	3 to 6 kWhe/m3	450 m3/h to 1166 m3/h	[111]
Stirling engine + TEC + single-stage flash desal	Dish solar collector 500 °C	506 W	3.5% (35,000 ppm)	N/A	28 kg/day	[98]
Solar still + TEG	Evacuated tube solar collectors 80–120 °C	1.4 W per TEG cell	N/A	N/A	0.97 kg/m2/h	[112]
Co3O4/NF hydrophobic membrane + TEG	Direct sunlight	0.74 W/m−2	3.5 wt% NaCl solution	N/A	1.76 kg/m2/h	[113]
ORC + ARCs + RO + LNG + TEG	Geothermal 160 K	ORCs 4.324 to 6.135 MW LNG 0.457 to 0.578 MW	Seawater 35,000 ppm	N/A	32,488 m3/day	[114]
Kalina cycle + RO + PEM + TEG	Geothermal water 230 °C	50 kW at 30 bar of geo-fluid pressure	45,000 ppm	N/A	~70 m3/h at 30 bar of geo-fluid pressure	[115]
CB/PVDF@BFP + TEG	Direct sunlight	~1600.43 mW/m2 under 1 sun	0.8–20 wt%	N/A	~1.41 kg/m2/h	[116]
Solar still + heat pipe + TEG	Photovoltaic panel 57.6 °C	75 W at 1 pm	N/A	N/A	1162 mL/m2	[117]
TEG coated by photothermal agent + bilayer nonwoven fabric + passive cooling vapor condenser	Direct sunlight	0.47 W	~102 to 104 mg/L	N/A	1.02 kg/m2/h	[118]
DBD plasma treatment + TEG	Direct sunlight	1.65 W/m2	3.5 wt% NaCl	N/A	Up to 1.82 kg/m2/h	[119]
Photothermal material + TEG	Direct sunlight	5.55 W/m2	~103 to 104 mg/L	N/A	12.1 kg/m2	[120]

HDH-D: humidification–dehumidification desalination; CB/PVDF@BFP: carbon black-/polyvinylidene fluoride-loaded bamboo fiber paper; DBD: dielectric barrier discharge.

summarizes the research on hybrid renewable energy technologies integrating power generation and desalination systems. Many researchers have focused on Kalina and HDH-D cycles coupled with TEG and renewable energy to produce fresh water between 0.586 m³/h and 32,488 m³/day. The range of electrical production has been between 0.47 W and 9.48 MW. Advanced systems, such as Brayton–Rankine cycles, with solar energy have shown to generate power from few kW to MW, while producing 450 m³/h to 1166 m³/h freshwater. Membrane-based technologies and other alternative technologies driven by solar energy are at much lower scales, producing freshwater up to 12.1 kg/m². The technologies showcase diverse temperature ranges (25–800 °C) and salinity capacities (up to 45,000 ppm), emphasizing sustainable solutions for the simultaneous generation of water and power.

4.5. Economic Feasibility

The economic feasibility of these hybrid systems varies significantly depending on the energy source, system configuration, and operational scale.

Table 6. Summary of power generation capacity, water production capacity, and costs of integrated system.

Technology (Study Type)	Power Generation Capacity	Water Production Capacity	Water Production Cost	Electrical Production Cost	Reference
SS-TEG-WI-M configuration	2.5 W	796 mL/m2/day	USD 0.071/kg (i.e., USD 71/m3)	-	[82]
Solar + geothermal + TEGs	9146.7 kW	240.226 m3/day	-	USD 0.037/kWh	[88]
Solar/biomass + multi-generation plant + TEG	33.03 MW	16.09 m3/h	-	USD 0.065/kWh	[89]
SOFC-GT	4.9 MW	85.2 m3/h	USD 0.223/m3	USD 0.029/kWh	[92]
ORC + MED	10 MW	38.51 m3/h	USD 0.4225/m3	USD 0.008087/kWh	[94]
Gasification combined cycle + MED	8.347 MW	11.7 kg/s	-	USD 0.051/kWh	[96]
Solar still + TEG	1.4 W per TEG cell	0.97 kg/m2/h	USD 0.0106/kg (i.e., USD 10.6/m3)	-	[112]
ORC + ARCs + RO + LNG + TEG	ORCs 4.324 to 6.135 MW LNG 0.457 to 0.578 MW	32,488 m3/day	USD 0.2205–0.3/m3	USD 0.0424–0.0521/kWh	[114]
Solar still + heat pipe + TEG	75 W at 1 pm	1162 mL/m2/day	0.042 USD/kg (i.e., USD 42/m3)	USD 0.061/kWh	[117]

SS-TEG-WI-M: solar still using TEG, iron scraps, and mirrors; SOFC-GT: solid oxide fuel cell–gas turbine; MED: multi-effect desalination.

presents a summary of product capacity and costs of the integrated systems.

The SS-TEG-WI-M configuration, a small-scale system, achieves a power output of 2.5 W and a water production rate of 796 mL/m²/day, and the authors have estimated the cost of water as USD 0.071/L [82]. In contrast, large-scale hybrid systems such as solar-, geothermal-, and TEG-based power plants reach a power capacity of around 9146 kW with a water production capacity of around 240 m³/day, achieving an electricity generation cost of USD 10.41/GJ [88], they do not estimate the cost of water.

Hashemian et al., presented their modeling analysis of a solar/biomass multi-generation plant with power generation of 33 MW, water production of 16 m³/h, and hydrogen production of 90.87 kg/h. For this scenario, they estimated the total cost rate of their product is USD 0.98/s and cost of power of USD 0.065/kWh [89]. Fuel cell-based hybrid systems are also investigated for the power and water production, as this generates the waste heat, along with electricity, at the desired temperature of desalination techniques [121,122,123]. For instance, the solid oxide fuel cell integrated with a gas turbine delivers 4.9 MW of power and 85.2 m³/h of water, with USD 0.223/m³ cost for water and USD 0.029/kWh for electrical power [92].

In comparison, organic Rankine cycle (ORC)-based systems, combined with multi-effect distillation (MED) with a 10 MW power capacity and 38.51 m³/h of water production, has a higher cost for water of USD 0.4225/m³, and a lower cost of electricity at USD 0.008087/kWh [94]. Similarly, a gasification-combined cycle with MED delivers 8.347 MW at 11.7 kg/s water production, with an electricity generation cost of USD 0.051/kWh [96].

Shafii et al. investigated a solar still integrated with TEGs with a power generation of 1.4 W per TEG cell, producing 0.97 kg/m²/h of water; at this small scale, the cost of water is significantly high, at USD 10.6/m³ [112]. Another study has shown that the cost of water can be very high when the scale of the system is very small, with 75 W electrical output and 1162 mL/m²/day of water production, the cost of water was estimated at USD 42/m³, while the cost of electricity was still reasonable, at USD 0.061/kWh [117].

So, it could be summarized that the cost of desalination is very sensitive to the system size as compared to the cost of electrical power production. Smaller-scale systems tend to have significantly higher water production costs due to limited economies of scale, whereas larger hybrid systems, particularly those integrating solar, biomass, and fuel cell technologies, achieve more cost-effective water production. Thermoelectric generators (TEGs), despite their ability to simultaneously produce electricity and utilize waste heat for desalination, currently face economic challenges at smaller scales, with significantly high water costs observed in TEG-integrated systems. However, TEGs remain an attractive option due to their solid-state nature, reliability, and potential for integration with solar stills and other renewable energy sources. Future advancements in TEG material efficiency, system integration strategies, and scale-up efforts could lead to more competitive cost structures. Additionally, optimizing the coupling of TEGs with multi-effect or membrane-based desalination techniques could enhance overall energy utilization and further drive down costs, making them a more viable option for sustainable water and power co-generation.

5. Discussion

The integration of thermoelectric power generation with water desalination technologies offers a promising solution to address the dual global challenges of energy and freshwater scarcity. The reviewed studies demonstrate innovative system designs that leverage renewable energy sources and waste heat to achieve simultaneous power generation and water desalination. Key advancements include the integration of thermoelectric generators (TEGs) with solar thermal systems, geothermal energy, and conventional power cycles, as well as the incorporation of desalination technologies such as reverse osmosis (RO), multi-effect distillation (MED), and humidification–dehumidification (HDH).

A significant trend across the literature is the effective utilization of low-grade renewable or waste heat in hybrid systems. For instance, Shoeibi et al. integrated TEGs with solar reflectors and thermal storage, increasing freshwater production by 24.4% and reducing costs [82]. Similarly, Assareh et al. optimized a combined solar and geothermal system using genetic algorithms, achieving improved system efficiency and a significant reduction in operational costs [88]. Mohammed et al. explored the use of organic Rankine cycles (ORCs) to recover waste heat for poly-generation systems, enhancing energy utilization by 50% and reducing production costs [94]. These examples underscore the adaptability of TEG-based systems to various thermal energy sources, making them suitable for decentralized applications.

While the studies reviewed demonstrate significant progress, several challenges persist. The low efficiency of TEGs remains a primary limitation. For example, Alzahmi et al. reported a TEG efficiency of only 5.5% at a temperature difference of 375 K [85]. Material properties, such as thermal conductivity and electrical resistivity, continue to constrain thermoelectric performance. Advances in nanostructured and high-ZT materials could help overcome these limitations, as suggested in studies focusing on material optimization.

Another challenge lies in the complexity of system integration. Systems combining multiple technologies, such as TEGs, traditional power cycles, and desalination systems, require precise design and control mechanisms. For instance, Demir et al. combined a Brayton cycle with TEGs and flash desalination, achieving high energy efficiency but highlighting the need for further optimization to address seasonal variations [84]. Similarly, Date et al. demonstrated that system performance can be enhanced under sub-atmospheric pressures, though additional work is needed to improve system robustness [83].

The implications of these advancements are far-reaching. Hybrid power generation and desalination systems can offer sustainable, decentralized solutions for energy and water supply, particularly in regions lacking centralized infrastructure. By utilizing renewable energy sources such as solar and geothermal energy, these systems reduce carbon emissions, lower costs, and contribute to global sustainability goals. For example, Hashemian et al. demonstrated the potential of a solar–biomass hybrid system to simultaneously produce hydrogen, potable water, and electricity [89], while Demir et al. highlighted the viability of waste heat recovery for multipurpose applications in marine systems [95].

The scalability and real-world applicability of combined power generation and desalination (CPD) systems remain critical considerations. While many studies demonstrate promising results at laboratory scale, such as Myneni et al.’s passive vacuum desalination system integrated with TEGs [86], scaling these technologies for commercial deployment presents challenges. Factors such as material costs, system complexity, and environmental conditions must be addressed to ensure consistent performance in diverse operational settings. For example, Khanmohammadi et al. investigated a large-scale Brayton cycle integrated with TEGs and reverse osmosis, highlighting the importance of optimizing working conditions, such as compressor pressure ratios and inlet temperatures, to balance energy and water outputs [87]. This underscores the need for pilot-scale demonstrations and long-term performance evaluations under varying real-world conditions.

Furthermore, the economic feasibility of CPD systems must be carefully considered alongside their technical performance. Studies such as those by Hashemian et al. [89] and Mohammed et al. [94] demonstrate that multi-objective optimization techniques can significantly reduce system costs while improving energy and water production efficiency. However, there remains a need for detailed life-cycle assessments and cost–benefit analyses that account for material choices, energy inputs, and environmental impacts. Integrating advanced tools like machine learning and artificial intelligence could play a crucial role in real-time optimization and cost reduction, enabling CPD systems to compete with conventional technologies. Additionally, policies promoting renewable energy adoption and waste heat recovery could provide the necessary incentives to accelerate the development and deployment of these systems in resource-limited settings.

While economic feasibility remains a critical consideration, the limitations of desalination technologies currently integrated with TEG systems further constrain their broader applicability. For instance, reverse osmosis (RO), while energy-efficient, relies on electrical energy to operate high-pressure pumps, limiting the direct utilization of thermal energy resources. Moreover, RO systems often face constraints in recovery ratios when dealing with high-salinity water sources. In contrast, thermally driven desalination systems, such as multi-stage flash (MSF) and multi-effect distillation (MED) technologies, suffer from large system footprints and high equipment costs due to their large and complex configurations. Similarly, humidification–dehumidification (HDH) systems, while modular and simple, demand significant space and material investment, particularly for heat exchangers and dehumidification units. These challenges highlight the need for alternative desalination approaches that overcome these limitations and align better with TEG-based CPD systems.

Membrane distillation (MD) presents a promising solution due to its compatibility with low-to-medium-grade thermal energy sources, compact configuration, ability to operate at high salinities, scalability, and lower complexity compared to conventional thermal desalination technologies. Addressing this gap through experimental studies, the optimization of coupling mechanisms, and system-level modeling could open opportunities for sustainable CPD solutions.

In summary, this review highlights the significant progress made in integrating thermoelectric power generation with desalination technologies. However, the lack of TEG-MD integration represents a major opportunity for future research. Future studies should focus on innovative TEG-MD coupling designs to develop efficient heat transfer pathways, and validating system performance under real-world conditions to assess scalability and economic feasibility.

6. Conclusions

In conclusion, the integration of power generation and water desalination technologies has gathered significant attention due to the pressing global challenges of increasing electricity demand and freshwater scarcity. This paper reviewed various advancements in thermoelectric generator (TEG) technologies, membrane distillation (MD) techniques, and their integration with power generation systems. The studies discussed highlight the potential of combining renewable energy sources, such as geothermal, solar, and biomass energy, with desalination processes to enhance system efficiency and sustainability.

Hybrid renewable energy systems integrate power generation and desalination, with power outputs from 0.47 W to 9.48 MW and freshwater production ranging from 0.97 kg/m²/h to 1166 m³/h. Large-scale systems achieve lower costs, such as USD 0.029–USD 0.065/kWh for electricity and USD 0.223–USD 0.4225/m³ for water, while small-scale TEG systems face high costs of up to USD 42/m³. Challenges include economic feasibility, energy variability, and heat recovery. Future research should enhance TEG material efficiency, integrate membrane-based desalination, and optimize scaling strategies. Advancing hybrid technologies can improve energy utilization, lower costs, and support sustainable water and power co-generation in diverse environments.

The integration of TEGs with other power generation and desalination technologies, such as photovoltaic modules, Stirling engines, and reverse osmosis systems, demonstrates the potential for simultaneous electricity and freshwater production. The performance of these integrated systems is influenced by various factors, including material properties, system configurations, and environmental conditions. For instance, the choice of working fluid in Stirling engines, the impact of solar radiation on system components, and the optimization of heat transfer and mass transfer processes play crucial roles in determining overall system efficiency.

Furthermore, the application of mathematical models and computational simulations has proven effective in predicting system behavior under different operating conditions, guiding the design and optimization of these integrated technologies. While significant progress has been made, challenges such as low thermoelectric efficiency, material limitations, and the complexity of system integration remain. Continued research and development are essential to overcome these challenges and realize the full potential of combined power generation and desalination systems in addressing the growing global demand for energy and water resources.